Electroless plating, also known as chemical or auto-catalytic plating, is a non-galvanic type of plating method that involves several simultaneous reactions in an aqueous solution, which occur without the use of external electrical power. Generally, the reaction is accomplished when hydrogen is released by a reducing agent and oxidized thus producing a negative charge on the surface of the part.
It is well-known that it is challenging to control film integrity and mechanical and thermal stability of thin films prepared by palladium thin-film deposition by electroless plating on microporous substrates. In particular, when the films are subjected to thermal cycling and prolonged operation at an elevated temperature and pressure, it may be problematic to control these characteristics of electroless deposited Pd/Pd—Ag thin-film on stainless steel substrates (Ilias, S., et al. (1997); Ilias, S. (1998); Ilias, S. (2001) and Ilias, S. (2006)).
In electroless plating deposition, the activation step may be crucial in fabricating palladium films. In general, pure and uniformly sparse palladium nuclei are required for catalytic deposition of palladium on porous surfaces. Usually, the sensitization/activation process helps to form a thin layer of atomic seed on the surface of the substrate to stimulate auto catalyzation prior to plating (Jost, W. (1969); Yeung, K. (1995); and Kikuchi, E. (1995)). In most conventional processes of fabrication of palladium membranes, the activation involves simultaneous oxidation-reduction reactions between palladium and oxidizing metal reagents, for example SnCl2/PdCl2. The simultaneous oxidation-reduction reaction introduces multifarious impurity of the palladium complex such as impregnated palladium hydroxide (Pd(OH)3), hydrated palladium (Pd-xH2O), palladium chloride or acetate (PdCL2, Pd(CH3COO)2), and poorly soluble hydrated stannous chloride (Sn(OH)1.5CL0.5). In the conventional electroless plating process, the nucleation and growth of palladium seed may locate only on a portion of the surface, which forms peel layers of coarse palladium particles. The uneven nucleation and growth of palladium seed may inhibit layer-to-layer overgrowth of palladium films on the substrate. Moreover, the deposited films may form severe lattice mismatching after long-term permeation exposure. Additionally, thermal stress may be developed between the substrate and deposited films, which may result in mechanical and thermal instability of the Pd-composite membrane (Uemiya, S. (1991)).
In the last decade, researches have shown that the stress in the polycrystalline deposited Pd-film prepared by conventional techniques such as an electroless plating process can be minimized by alloying with others metals. Recently, it has been reported that an inter-metallic diffusion layer, for example, oxide layer, is used to fabricate Pd—Cu alloy membranes on stainless steel substrates and there is some success in thermal stability of the membranes. (See Ma, Y. H., and Pomerantz, N., 2006 UCR Contractors Review Conference, Pittsburgh, Abstract pp. 15-16, (2006)).
However, in view of limited methods of fabricating thin films prepared by electroless plating, there is a significant need for an improved method to prepare thin films by electroless plating.